i dont know but it might help.
I do cheat a bit when it comes to surrendering.. when one race surrender i load autosave straight away... attack that race and offer her a "Peace Treaty" demanding few star systems (usually i demand the most populated ones and/or the smaller ones, with bigger ones in same sector). I know that the race soon is going to surrender and obviously it cultural rate is not that high to compit with yours.. so firstly you are going to see little pyrate sign and then hopefuly that planet is going to convert..
Of cource demanding star systems requires a very high diplomacy level that ussuallu comes after i research Majesty III , GOD THAT LITTLE BABY IS GOOD,, it can do magic
P.C. I know that all was a bit of shore but thats what i do when it comes to surrendering :notsure: